With the development of mobile communication technology, mobile communication networks are evolving generation by generation. Currently, a mobile communication network has a form in which a synchronous mobile communication system (CDMA mobile communication system), called a 2nd or 2.5th generation network, and an asynchronous mobile communication system (WCDMA mobile communication system), called a 3rd generation network, coexist.
With the development of such mobile communication network technology, a mobile communication terminal (Dual Band Dual Mode Terminal; DBDM mobile communication terminal), capable of being used both in a synchronous type system and in an asynchronous type system, is being developed. Using such a mobile communication terminal, different types of services can be used in the respective areas of an asynchronous type system and a synchronous type system. Such a DBDM mobile communication terminal includes an antenna, a synchronous modem unit for performing synchronous mobile communication, an asynchronous modem unit for performing asynchronous mobile communication, and a common module.
Meanwhile, since the asynchronous mobile communication system is at an early stage of service and cannot provide service over a wide area due to the massive investment cost for the implementation of the system, the asynchronous mobile communication system is implemented to overlap the area of a synchronous mobile communication system. As a result, the service area of the asynchronous mobile communication system is limited, so there is a problem in that a call is disconnected in the case where subscribers to the asynchronous mobile communication system move to a synchronous area, in which an asynchronous mobile communication service is not provided, while using a service in an asynchronous area.
As described above, in the case where an asynchronous mobile communication system and a synchronous mobile communication system coexist and the area of the asynchronous mobile communication system is smaller than that of the synchronous mobile communication system, handover is required to provide continuous service between the asynchronous and synchronous mobile communication systems. Furthermore, such handover needs to be supported during the use of the voice communication service of a mobile communication terminal or a reception alerting state, or while waiting for the response of a called party after the origination of a call.